The invention is based on a directional control valve which is suitable for the load-independent control of the direction and speed of a hydraulic load.
A directional control valve is disclosed by German Offenlegungsschrift 23 42 498. It has a control slide which is axially displaceable in a slide bore in a valve housing and, in a neutral position, shuts off two load chambers on the slide bore from a feed chamber and, in two operating positions on either side of the neutral position, selectively connects one of the two load chambers to a feed chamber and connects the respectively other load chamber to a return chamber. In each of two accommodation chambers, which are located opposite each other and are made in the control slide from the opposite ends thereof, there is arranged a brake piston, which opens a flow path between a load chamber and a return chamber in each case only to such an extent that the hydraulic load can no longer lead the amount of fluid flowing toward it, that is to say that no lack of filling is produced in the flow line between said hydraulic load and the other load chamber. For this purpose, the brake piston is acted on by a spring with the effect of reducing the opening cross section of the flow path, and is acted on by the pressure in the other load chamber with the effect of enlarging the opening cross section. For the application of pressure, the control slide has a longitudinal slide bore, which can be connected via a transverse bore to the other load chamber and which opens into a pressure chamber between the bottom of an accommodation chamber and the brake piston located in this accommodation chamber. The longitudinal bore runs eccentrically in the control slide. In order to apply pressure to the other brake piston, there is a further longitudinal slide bore running eccentrically in the control slide, and a further transverse bore.
In the directional control valve disclosed by German Offenlegungsschrift 23 42 498, two feed chambers run around the slide bore at a distance from each other. The control slide has two annular grooves which are spaced apart axially from each other and of which each is respectively used to connect one of the two feed chambers to a load chamber. In the piston collar between the two annular grooves, the control slide has two further, narrower annular grooves which are separated from each other by a narrow web. One longitudinal slide bore is connected to the one narrow annular groove by a first transverse bore, and the other longitudinal slide bore is connected to the other narrow annular groove by a second transverse bore. A load indicator channel, which in the neutral position of the control slide is covered by the narrow web between the two narrow annular grooves, opens into the slide bore. Depending on the direction in which the control slide is moved from the neutral position, the load indicator channel is opened toward one or the other annular groove, and hence the load pressure prevailing in the respective load chamber is indicated in the load indicator channel.
While the directional control valve according to German Offenlegungsschrift 23 42 498 has two feed chambers, and a load indicator channel opens into the slide bore between these two feed chambers, the brochure 9129 8557-02 (GB) from Voac Hydraulics AB discloses a directional control valve, designated K170LS, in which, in a slide bore, a return chamber, a load chamber, a single feed chamber, another load chamber and a further return chamber follow one another, and a load indicator channel opens into the slide bore laterally at a point, that is to say, as viewed from a load chamber, on the other side of the adjacent return chamber. Using such an arrangement, it is possible for the directional control valve to be shorter in the axial direction of the control slide than that from German Offenlegungsschrift 23 42 498. In the directional control valve according to the above-mentioned brochure, a longitudinal bore which is closed on both sides runs centrally through the control slide, and three transverse bores or transverse bore stars lead from said longitudinal bore to the outside of the control slide. In the neutral position of the control slide, a first transverse bore star is located at a short distance from a load chamber, between the latter and the feed chamber. The second transverse bore star is located at a short distance from the other load chamber, between the latter and the feed chamber. Finally, a third transverse bore opens into an annular groove on the outside of the control slide and, in the neutral position of the control slide, is located symmetrically in relation to the mouth of the load indicator channel and, in every position of the control slide, connects the longitudinal slide bore to the load indicator channel.